This invention pertains to the field of electrically sensing the speed of rotating shafts, and more specifically to the magnetic pulse exciter for use with magnetic speed sensing pickup elements used with conventional electronic digital tachometers.
Magnetic pickups are used with various types of electronic tachometers and counting devices for speed and/or count sensing. These pickups generate an electrical pulse to each time a piece of magnetic material passes close to the pickup head. When used in determining the speed of a rotating shaft, a gear is normally affixed to the shaft in such a manner that the gear teeth pass close to the magnetic pickup head as the shaft is rotated. Each gear tooth passing the head then generates an electrical pulse. The rotational speed of the shaft can then be determined from the knowledge of how many teeth are on the specific gear and the rate of the pulses generated. The gear teeth or other magnetic material passing the magnetic pickup are usually referred to as the magnetic exciters.
The problem solved by the present invention is one of placing a magnetic exciter on installed shafts of various diameters without dismantling the shafts, without using precisely machined split gears attached to the shafts, and without using indirect drives to rotate auxiliary pulse generating exciters.
In determining the speed of a slowly turning shaft with a magnetic pickup, uniform spacing of the magnetic exciters around the shaft is desired so that a constant pulse rate is obtained when the shaft is rotating at a fixed RPM. This allows the pulse rate sample time (the period for which the pulses are counted) to be short, allowing a fast response of the electronic readout while still providing an accurate speed indication. If the exciters were not spaced uniformly, more pulses would be counted in one sample period and less in the next as the shaft is rotated. This uneven spacing would therefore produce a changing speed indication because of the varying pulse count, even though the shaft would be turning at a fixed RPM. Longer sample times, counting pulses for many revolutions of the shaft, reduces the errors caused by uneven exciter spacing. Longer sample times are not desirable in many applications as they proportionately slow down the response of the readout to actual changes in speed. Also, it is desirable that a fixed number of these exciters be spaced around the shaft so that the electronic readout unit can have a fixed calibration based on a fixed number of pulses per revolution of the shaft.
In the application of reading shaft RPM where accuracy, fast response and no field calibration of the readout is desired, the magnetic exciters must be of a fixed number with uniform spacing. Shaft diameters have a wide range of variation due to horsepower ratings of engines and specific applications. To accommodate magnetic pickups on varying size shafts, split gears of a known number of teeth have been precisionally machined and mounted to the shafts. A second current method is to affix a belt drive from the shaft to a separate pulley of the same diameter as the shaft. This pulley ten drives the magnetic exciter gear of a fixed number of teeth. These methods require a precision gear or pulley machined and fabricated for each individual shaft diameter.
Finally, while magnetic pickups are capable of providing speed signals to tachometers, the direction of shaft rotation usually must be obtained by different means, because the magnetic pickup is insensitive to direction of shaft rotation.